Civil Engineering Reference
In-Depth Information
In the absence of an oxidant, manganese is removed only via adsorption onto the
manganese dioxide medium. The chemical reaction for oxide-coated media is listed
in Table 14-3. Knocke and coworkers
12
reported no evidence of a chemical reaction
between manganese and MnO
2
media. Thus, if the Mn(II) is not eventually oxidized,
the adsorption sites on a medium will ultimately be used up and the Mn(II) will pass
through the filter. Alternatively, if there is a change in water chemistry, the Mn(II)
could be released from the filter into the finished water. It is for this reason that either
ClO
2
,Cl
2
, KMnO
4
,orO
3
is added to water being applied to manganese dioxide media
filters. Use of one of these oxidants will convert the Mn(II) that is attached to the
media grains to its insoluble form, Mn(IV). This freeing up of the adsorption sites on
the media is referred to as
regeneration
. The insoluble Mn(IV) is then removed during
backwashing of the filter. To maximize the benefits derived from using these oxidants,
care must be used in the selection of the oxidant and in establishing the proper dosage.
The pH of the water and the number of adsorption sites available on the media
surface have a significant impact on the manganese adsorption capacity of the man-
ganese dioxide ore (or, for that matter, manganese dioxide coated media). Knocke and
coworkers
12
reported that for pH values of less than 6.0 minimal Mn(II) was noted.
Above a pH of 7.0, the number of adsorptive sites available increased significantly.
Manganese Greensand
Alternative media have been developed to take advantage
of the adsorptive capacity of manganese dioxide while eliminating the hydraulic prob-
lems associated with the medium's heavy weight. Manganese greensand is a product
manufactured by treating New Jersey glauconite with manganous sulfate and potassium
permanganate to provide an active supply of iron and manganese oxides on the sand
grains. When the oxidizing power of the bed is exhausted, the bed is regenerated with
permanganate or chlorine and returned to service. These beds are quite effective in
high-carbonate, iron-bearing water, but they are exhausted or fouled quickly if other
reducing substances, such as organic matter, nitrogenous matter, or hydrogen sulfide,
are present. Greensand has the ability to adsorb and then catalyze the oxidation of
iron and manganese and then provide filtration. Because of greensand's small size,
hydraulic loading rates must be kept low; the filter bed can bind off quickly, resulting
in short filter runs. When the amount of precipitate is large, a layer of crushed an-
thracite coal is usually placed over the exchange medium to reduce the loading on the
greensand and thus prolong filter runs.
Because greensand media regeneration is time-consuming, continuous regeneration
is commonly employed at plants in North America. During continuous regeneration,
a solution of KMnO
4
is fed continuously into the raw-water line ahead of the filter to
reduce the amount of soluble iron and manganese applied to the filter. The dose of
KMnO
4
is critical. Underdosing results in manganese breakthrough, and overdosing
produces pink water.
Manganese Dioxide Coated Silicon Dioxide
Another oxide-coated medium that
has been used for the removal of iron and manganese is manganese dioxide coated
silicon dioxide. Known by the trade name ''Birm,'' this medium was developed in the
1930s and first used in 1939. The primary use of manganese dioxide coated silicon
dioxide is for iron removal. The unit weight of the medium is only 50 lb / ft
3
(800 kg/
m
3
), which reduces the required backwash flow to 10 to 12 gpm / ft
2
(24 to 29 m / h).
The adsorptive and catalytic capacity of the medium is not used up during operation
and requires only frequent backflushing to remove the accumulated oxidized metals.
